RepRapPro Huxley Duo Z Homing doesn't work

I got a RepRapPro Huxley Duo kit last year, and built it then successfully and did a couple of prints, but haven't used it since because of other reasons.

The printer always had a problem where the IR sensor for measuring the Z axis height doesn't do so, instead continuing to lower the axis when homing, to the point of snapping one of the parts. The sensor works absolutely fine for doing the X axis bit, so I don't see why it should be any different. There's something in the instructions about putting paper under the kapton tape to help the sensor reflect, but apparrently (see this post here), it's not needed for the Huxley due to the fact that it's got an aluminium heat bed. I haven't even got to the point of applying kapton tape yet, so I really can't see how it is reflecting off anything. RepRapPro has now gone out of business, so this is my only option for support! Their instructions seem to be just copied and pasted from the Ormerod 2 ones anyways.

Why is this going wrong, and how can I fix it? I don't want to buy any more parts if I can avoid it. Thanks!

If the Huxley config.g and homing files are like the Ormerod ones, then it uses the proximity sensor in unmodulated mode (M558 P1) when X homing, and modulated mode (M558 P2) when Z homing. You can check this by looking at the homeall.g file. If the modulation isn't working because the T wire is connected incorrectly or the proximity sensor is faulty, it could cause that problem.

I suggest you try sending M558 P2 and then raise a piece of white paper towards the nozzle while monitoring the Z probe value in the web interface. You should see two numbers, with the second one in brackets. As you raise the paper, the first one should increase, but the second one should stay low.

With M558 P1 instead, there will be only one number displayed, which will increase as you raise the white paper.

Have I understood correctly that the Huxley Duo has an aluminium bed plate without glass on top?

What G31 P and Z parameters do you have set in config.g, and how did you establish those values?

Your G31 Z parameter needs to be positive. Use a much lower P value such as 400 to achieve this. Also check that the black IR sensor at the bottom of the proximity board has not been bent out of position, and it is no more than about 1.5mm higher than the tip of the nozzle.

I think that I do perhaps need the paper. When about a millimetre away from the aluminium plate, the G31 command reports back values of about 150, but if I put a piece of paper under it in the same place, I get values like 658. This happens when the build plate has kapton tape on it, and when it doesn't. I think I'll just stick paper over the kapton tape for a stop-gap measure, but according to RepRapPro this shouldn't be necessary... Has anyone else had this experience?

1. If it's like the Ormerod, the proximity sensor screw holes are somewhat larger than the M2.5 screws, so you can slide the sensor board down a little before you tighten them.

2. From your photo, it looks to me that the IR sensor (the black rectangle at the bottom of the board) may have been bent up slightly so that it is not parallel to the bed. If that is the case, bend it back into position.

3. Check that the print head is assembled correctly. On the Ormerod, people occasionally fitted the nozzle mount the wrong way round, which placed the nozzle too low relative to the sensor.

btw I make an improved IR sensor for bed levelling, see [miscsolutions.files.wordpress.com]. It's not mechanically compatible with the RRP sensor, so you would need a new printed part to use it.